Biochemical assays such as immunoassays (e.g., enzyme-linked immunosorbent assay (ELISA)) and methods for detecting nucleic acid sequences in a test sample are well known. In such assays, high sensitivity is important to ensuring the ability of the assay to detect low levels of the analyte of interest. Radioactive and colorimetric methods have often been employed in such assays. However, achieving high levels of sensitivity has not always been possible, and methods of increasing the sensitivity of such tests are desirable.
One reported method (European Patent Publication EP 128 332) for detecting analytes, including nucleic acid sequences and antigens (or antibodies), is the use of a "bridging moiety," which provides a bridge between an analyte and a "signalling moiety" which provides a detectable signal. The bridging moiety includes an analyte-specific region and a signalling moiety-specific region. This method has the advantage that, by varying the bridging moiety according to the target analyte, the same signalling moiety can be employed to detect a variety of analytes. However, the preparation of the bridging moieties can be rather lengthy and inefficient. Also, large bridging moieties (such a long polynucleotide sequences) may be less sensitive at detecting target analyte due to the presence of large segments which do not bind to the target. Moreover, this method requires that the analyte-specific region and a signalling moiety-specific region of the bridging moiety be different.
Another publication (U.S. Pat. No. 5,627,030 to Pandian et al.) describes the use of a primary probe for detecting a target nucleic acid sequence, and an "amplification probe", which is a nucleic acid sequence which includes two regions: a first region complementary to the primary probe, and a second region which contains repeated sequences for binding to a plurality of labeled probes. The amplification probe permits several labeled probes to bind to each primary probe, amplifying the signal from the binding of the primary probe to the target molecule in the sample. However, construction of the amplification probe can be cumbersome.